1. Field of the Invention.
The present invention relates to a device for creating a notch in a waveform supplied by an alternating power source to a load and more particularly to a load comprising an inductive ballast for a fluorescent light to provide for the dimming thereof.
2. Reference to Other Applications
Two other patent applications filed in the name of the present inventors and assigned to the present assignee, entitled "Power Control For Inductive Loads" and "Time Delay Initialization Circuit," respectively, have been filed on even date herewith and disclose and claim circuits useful in cooperation with the present invention.
3. Description of the Prior Art.
In a co-pending application Ser. No. 898,569, filed Aug. 21, 1986, in the name of L. S. Atherton, R. A. Black, Jr., and A. D. Kompelien, and assigned to the assignee of the present invention, a circuit is described for use with a fluorescent light system to accomplish the dimming thereof by creating a "notch" in the waveform supplied by an alternating power source. The position and width of the "notch" are selected so as to reduce the power supplied to the fluorescent light ballast and thereby accomplish dimming thereof.
In this co-pending application, a pair of power diodes are connected with their cathodes connected to the power source and the load, respectively, and with their anodes connected together to a common node. A pair of unidirectional switches such as "gate turn-off" thyristors (GTO's) are also connected between the power source and the load. A first GTO is connected between the source and the node so as to conduct current from the source to the node during positive half cycles of the alternating supply providing that the GTO is "on". The second GTO is connected between the load and the node so as to conduct current from the load to the node during negative half cycles of the supply providing the GTO is "on". The GTO's are turned "on" during the majority of a cycle from the power source by a control signal being a positive signal applied to the control input or gate thereof from a control circuit. Accordingly, at the beginning of a positive half cycle, current flows from the power source through the first unidirectional switch to the node and from the node through the first power diode to the load. Likewise on the beginning of a negative half cycle, current flows from the load through the second unidirectional switch to the node and from the node through the first power diode to the source.
When it is desired to cut a "notch" in this waveform, the signal to the control electrodes or gates of the unidirectional switches are made negative so that the GTO's turn "off" and current no longer flows in the above-described path, and the load receives no current.
Since GTO's cannot be turned back "on" without consuming considerable switching power, the previous circuit also included a pair of silicon controlled rectifiers (SCR's) connected between the source and the node and one between the node and the load. When turned "on" by a control signal at the gate of each SCR, they conduct current in the same manner as the GTO's, i.e. from the source or the load to the node and then through the power diodes. Accordingly, at the end of the "notch" the first SCR is actuated by a control signal so that current could flow through the SCR to the node and through the power diode to the load for the remaining portion of the positive cycle. At the zero crossover, the silicon-controlled rectifier is automatically turned "off", but by this time the appropriate GTO is turned back "on" and current now flows through the switch and the power diode until the next "notch". At the end of the "notch" in the negative half cycle, the second SCR is turned "on" and current flows from the load through the SCR to the node and then from the node through the power diode to the source for the rest of the half cycle. "Notches" are preferably cut in both the positive and negative half cycles because otherwise a DC bias signal is created which causes undesirable flickering in the fluorescent lamp.
One problem associated with the above-described circuit is that after the notch has been created, the silicon-controlled rectifiers continue to conduct current for the remaining portion of the half cycle, and all of this current flows through the power diodes which consume a large amount of power and produce excessive heat.